Particle accelerators for producing high energy charged particle beams are used for basic physics research and medical applications. The key component of an accelerator is a slow wave structure, which provides an interactive space for radio frequency (rf) fields to interact with the charged particles for acceleration. In order to accumulate the acceleration effect, the phase velocity of the rf fields must synchronize with the particle beam velocity. Therefore, the first specification of a slow wave structure is its phase velocity, v.sub.p as a function of frequency (or equivalently the slow wave ratio SWR=c/v.sub.p, where c is the speed of light in the free space). In order to enhance the interaction of the rf fields and particles, the rf electrical field must be sufficiently high along the particles' beam path to produce a strong force for efficient acceleration. Therefore, the second specification of a slow wave structure is a parameter called the coupling impedance, Z.sub.c, defined as: ##EQU1## where P is the dissipated power in one section of the slow wave structure. ##EQU2## is the E-field line integration along the particle path where E is the electrical field strength and d1 is the differential line element at path of the charged particle beam and along the longitudinal slow wave structure; and L is the length of the section of the slow wave structure. Coupling impedance Z.sub.c can be expressed as EQU Z.sub.c =Q.sub.0 G (3)
where Q.sub.0 is the unloaded Q-value of the structure and G is defined as the geometry factor: ##EQU3## where f.sub.0 is the resonant frequency of the resonator and where W.sub.0 is the stored energy in the resonator at the resonant frequency.
A dc high voltage, V.sub.o, can be used to accelerate the charged particle beam to an initial "injection" velocity, v, fed into the slow wave structure. The non-relativity relation between V.sub.o and v is: ##EQU4## where v, e and m are the velocity, electrical charge and the mass of the particles, respectively. Unless very high dc voltage is used, v is much less than the speed of light c, which means that the slow wave ratio should be much greater than unity at the entry sections of the slow wave structure and should gradually decrease to keep synchronized with the accelerated particle beam.
Slow wave structures are also used in traveling wave tubes (TWTs). Contrary to the accelerator case, the electron beam in a TWT is decelerated to tranfer energy to the rf fields for amplification. Such interaction also requires synchronization between the electron beam velocity, v, and the phase velocity, v.sub.p, of the rf fields. The difference is that, in the accelerator case, v is less than or about equal to v.sub.p, whereas in the TWT case, v is greater than or about equal to v.sub.p.
The conventional slow wave structures have a tubular shape and are made of a common metal, such as copper, with a periodic structure along the longitudinal direction. These structures also can be viewed as a series of coupled resonant cavities. The phase velocity and the coupling impedance can be adjusted by varying the dimensions of the resonant cavities, or varying the coupling between the cavities. The main problem with these conventional metallic slow wave structures is the low coupling impedance, Z.sub.c, due to the low Q-value. The low Z.sub.c causes a low efficiency, which must be compensated for by increasing input rf power and using a longer slow wave structure. Both measures are costly. One way to solve the problem is the use of a low temperature superconductor (LTS) such as niobium (Nb) or lead (Pb) to replace the normal metal used in making the slow wave structure. Such LTS slow wave structures have extremely high Q-values, e.g., up to 10.sup.9, which greatly increases the Z.sub.c and thereby improves the efficiency. However, the LTS structures must be operated at or near liquid helium temperature (4.2 .degree.K), which drastically complicates the overall structure and increases the cost. Except for some very special cases, the cost of operation of most acelerators at such a temperature cannot be justified.
The present invention overcomes the above-discussed problems by providing an HTS/dielectric slow wave structure operated at or near liquid nitrogen temperature (77 .degree.K) with an extremely high Q-value. It provides an adjustable slow wave ratio suitable for accelerators and TWTs which improves their efficiency and shortens the length of the slow wave structure resulting in more compact accelerators.
Commonly assigned, copending application Ser. No. 07/788,063, filed Nov. 5, 1991, (now U.S. Pat. No. 5,324,713 issued Jun. 28, 1994) describes an HTS/dielectric TE.sub.0in (i and n=1,2, . . .) mode resonator. Several TE.sub.011 mode HTS/sapphire resonators described therein demonstrated extremely high Q-values up to 3.times.10.sup.6 and power handling capability up to 3.times.10.sup.4 watts at 80K. This experimental data proved that thin film HTS materials, such as YBaCuO, TlBaCaCuO, and dielectric materials, such as single crystal sapphire (.alpha.-Al.sub.2 O.sub.3), are capable of achieveing extremely high Q-values at microwave frequencies for high power applications. However, such TE mode resonators do not have an E-field along the longitudinal direction, which is required by slow wave structures to interact with a charged particle beam. The present invention overcomes this problem by providing an HTS/dielectric structure formed by a series of TM or EM mode HTS/dielectric resonators, as described below in reference to FIGS. 1a-1b, which have all the characteristics required by a slow wave structure. The structures in accordance with this invention can greatly increase the accelerator's efficiency and make it more compact.